Atilla Incecik

Experimental study of slam-induced stresses in a containership

Experiments for the ship motions and sea loads were carried out on a segmented model of a container ship in ballast condition. Comparisons between the measurements and the theoretical results were carried out for the vertical motions and bending moments. For the evaluation of the primary stresses it is assumed that the total vertical bending moment induced by waves is divided into one component obtained by the linear theory and another one is due to the slamming loads. Several formulations for the determination of the slamming loads are compared with experimental results. The vibratory response of the model is calculated by modelling the hull with rotational springs and rigid links. Linear finite elements with a consistent mass formulation are adopted for the structural model and the response is obtained by modal superimposing and direct integration methods.

Analytical solutions of the diffraction problem of a group of truncated vertical cylinders

Abstract An exact analytical method is described to solve the diffraction problem of a group of truncated vertical cylinders. In order to account for the interaction between the cylinders, Kagemoto and Yues exact algebraic method is utilised. The isolated cylinder diffraction potential due to incident waves is obtained using Garrets solution and evanescent mode solutions are derived in a similar manner. Numerical results are presented for arrays of two and four cylinders. Comparisons between the results obtained from the method presented here and those obtained from numerical methods show excellent agreement.

An experimental investigation of motion control devices for truss spars

This paper presents the results of an experimental study carried out on a model truss spar in regular and irregular waves. Each bay of the truss of the model was spanned by a horizontal plate. The bottom bay was instrumented so that the vertical loading could be measured. Four different types of plate were fitted to the model and the responses of the spar to the wave loading were recorded for each type. Two of the plate types were perforated and two were solid. The response amplitude operators, and the force coefficients in the vertical direction, are presented and the performances of the different types of plate are assessed.

Global wave loads on intact and damaged Ro-Ro ships in regular oblique waves

A nonlinear time-domain simulation method is presented for the prediction of dynamic global wave loads on a Ro-Ro ship at zero speed in regular oblique waves in an intact and a damaged condition. Numerical computations and model tests have been carried to investigate the structural responses of Ro-Ro ship Dextra to various wave amplitudes at three different wave headings (DTR-4.1-NEW-12.98, DEXTREMEL project BE97-4375, 1998; DTR-4.2-NEW-11.99, DEXTREMEL project BE97-4375, 1999). The results of numerical and experimental investigations for stern quartering waves are reviewed. Comparisons between predictions and measurements for global wave loads at the midship section of the intact and the damaged Ro-Ro ship show that the agreement between the theory and experiment for dynamic horizontal and vertical bending moments is excellent. On the other hand, correlation between the predictions and measurements for dynamic vertical shear force is better than that for dynamic horizontal shear force. Nevertheless, the calculated torsion moment values are higher than the measured values. As the wave amplitude is not small, the positive and negative peaks of global wave loads are no longer equal to each other as found in both the calculations and experiments. The dynamic vertical global wave loads in the damaged condition are larger than that in the intact condition.

Global wave loads on a damaged ship

A computational tool was applied based on a two-dimensional linear method to predict the hydrodynamic loads for damaged ships. Experimental tests on a ship model have also been carried out to predict the hydrodynamic loads in various design conditions. The results of the theoretical method and experimental tests are compared to validate the theoretical method. The extreme wave-induced loads have been calculated by short-term prediction. For the loads in intact condition, the prediction with a duration of 20 years at sea state 5 is used, while for loads in damaged conditions, the prediction with 96 hours of exposure time at sea state 3 is used. The maximum values of the most probable extreme amplitudes of dynamic wave-induced loads in damaged conditions are much less than those in intact condition because of the reduced time. An opening could change the distribution of not only still-water bending moment but also wave-induced bending moment. It is observed that although some cross-sections are not structurally damaged, the total loads acting on these cross-sections after damage may be dramatically increased compared with the original design load in intact condition.

Wave enhancement due to blockage in semi-submersible and TLP structures

An exact analytical method is described to solve the diffraction problem of a group of truncated vertical cylinders. In order to account for the interaction between the cylinders, Kagemoto and Yues exact algebraic method is utilised (Kagemoto, H., Yue, D.K.P., 1986. Interactions among multiple three-dimensional bodies in water waves: an exact algebraic method. J Fluid Mech, 46, 129-139). The isolated cylinder diffraction potential is obtained using Garrets solution and evanescent mode solutions are derived in a similar manner (Garret, C.J.R., 1971. Wave forces on a circular dock. J Fluid Mech, 46, 129-139). Free surface elevations are calculated for an array of four cylinders and compared with experiments. Comparisons show good agreement.

INVESTIGATION OF MOTIONS OF CATAMARANS IN REGULAR WAVES-I

By extending the linear frequency domain theory, a quasi-non-linear time-domain technique has been developed to investigate the large amplitude motions of catamarans in regular waves. The non-linearity of hydrodynamic forces included in this practical method comes from variations of a ships submerged portion. These forces are obtained from a database generated by the linear frequency domain method at each time step. The coupled equations, heave and pitch, are solved in the time domain by using the Runge-Kutta method with proper initial values. In order to investigate the non-linear effects of large amplitude motions of the V-1 catamaran in the head-sea condition, numerical results obtained from the linear and non-linear strip methods have been compared with those obtained from a series of experiments carried out in the towing tank of the Hydrodynamics Laboratory at the University of Glasgow. Based on the comparative studies, the numerical results obtained from the time-domain program can provide better predictions for the large amplitude motions of catamarans than the linear frequency domain method. It is concluded that the non-linear effects are significant when the model speeds and wave amplitudes increase. The peak values of large amplitude motions around the resonance frequencies, as obtained from the non-linear time-domain predictions as well as from measurements, are smaller than those obtained from the linear theory.

Investigating the reliability and criticality of the maintenance characteristics of a diving support vessel

Maintenance tasks and their application in the shipping industry have evolved significantly in the recent years. Particularly in the offshore industry, safety onboard, environmental protection and intensive operational activities necessitate the minimization of down-time and the preservation of an excellent performance ratio. The first step of an innovative ship maintenance strategy, which is proposed by the authors and is based on criticality and reliability assessment, is presented herein using the FTA tool with time-dependant dynamic gates so as to represent in an accurate and comprehensive way the interrelation of the components of a system. The paper also presents a review of the maintenance standards and procedures, such as the ALARP concept, the Key Programme 3-Asset Integrity (KP3) initiative, the OREDA handbook as well as the RCM and RBI principles. As part of the reliability assessment, the Birnbaum and Criticality reliability importance measures are utilized to validate the results of the analysis. A case study of a diving support vessel (DSV) illustrates the application of this strategy. The main systems examined are: the vessels power plant, propulsion, water system, lifting, hauling and anchoring, diving and finally the safety system. The reliability of the main systems and subsystems as well as of their critical components is identified and suggestions of how to improve the overall reliability of the various systems both at a component, system and managerial level are also proposed.

Scenarios for the assessment of the collision behavior of ships

Since the late fifties, when the first nuclear powered merchant ships were designed, collision research has been focused on the establishment of models for the prediction of the damage of the side of a ship involved in a ship-ship collision. Such models have been used to assess the performance of the struck ship in case she is involved in a collision. Further it was and it is still highly desirable to develop design tools which could provide guidance for the construction of ship hulls which will minimize as far as possible the risk in case of a collision. In order to assess the collision behaviour of a ship, there is a need to test her under specific collision incidents, which are described by a number of parameters, such as the particulars of the striking ship including the stiffness of her bow, the speeds and relative orientation of the two vessels, sea conditions, etc. The assessment methodologies are deterministic, i.e. they are based on one or a few severe collision scenarios, or probabilistic, i.e. they consider distribution functions of one or more collision parameters. The paper presents and discusses existing codes concerning collision scenarios for floating and fixed marine structures, presents data that can be used for the determination of the level of the loading that a ship may experience if she is involved in a collision and gives examples of relevant calculations.

EXTREME MOTION RESPONSE ANALYSIS OF MOORED SEMI-SUBMERSIBLES

The motion response prediction of offshore structures may be carried out using time domain or frequency domain models or model tests. The frequency domain analysis uses the simplified, linearised form of the motion equations and it is very economical. The time domain analysis, unlike frequency domain models, is adequate to deal with non-linearities such as viscous damping and mooring forces, but it requires sophisticated solution techniques and it is expensive to employ. For moored semisubmersibles time domain techniques must be employed since there are strong nonlinearities in the system due to mooring line stiffness and damping and viscous drag forces. In the first part of this paper a time domain model to predict the dynamic response of a semi-submersibles are developed and the effect of thrusters and mooring line damping are incorporated into the time domain model. In the second part time domain simulations are carried out to find the total extreme motions and mooring forces.